Method of restoring xerographic properties to a glass binder plate



United States Patent 3,288,603 METHOD OF RESTORING XEROGRAPHIC PROP-ERTIES TO A GLASS BINDER PLATE Lester Corrsin, Penfield, N.Y., assignorto Xerox Corporation, Rochester, N.Y., a corporation of New York NoDrawing. Filed Apr. 29, 1964, Ser. No. 363,609 Claims. (Cl. 96-4) Thisapplication is a continuation-in-part of my prior application, SerialNumber 184,594, filed April 2, 1962, now Patent No. 3,151,982.

This invention relates in general to xerography and in particular toxerographic plates.

In the xerographic process as described in US. 2,297,- 691 to C. F.Carlson, a base plate of relatively low electrical resistance such asmetal, paper, etc., having a photoconductive insulating surface coatedthereon, is electrostatically charged in the dark. The charged coatingis then exposed to a light image. The charges leak off rapidly to thebase plate in proportion to the intensity of light to which any givenarea is exposed being substantially retained in non-exposed areas. Aftersuch exposure the coating is contacted with electrostatic markingparticles in the dark. These particles adhere to the areas where theelectrostatic charges remain, forming a powder image corresponding tothe electrostatic image. The powder image can then be transferred to asheet of transfer material, resulting in a positive or negative print,as the case may be, having excellent detail and quality. Alternatively,where thebase plate is relatively inexpensive, as of paper, it may bedesirable to fix the powder image directly to the plate itself.

As discussed in Carlson, photoconductive insulating coatings compriseanthracene, sulfur or various mixtures of these materials such as sulfurwith selenium, etc., to thereby form uniform amorphous coatings on thebase material. These materials have a sensitivity largely limited to theshorter wave lengths and have a further limitation of being onlyslightly light-sensitive. Consequently, there has been an urgent needfor improved photoconductive insulating materials.

The discovery of the photoconductive insulating properties of highlypurified vitreous selenium has resulted in this material becoming thestandard in commercial xerography. The photographic speed of thismaterial is many times that of the prior art photoconductive insulatingmaterials. However, vitreous selenium suffers from two serious defects:(1) its spectral response is very largely limited to the blue or nearultra-violet; and (2) the preparation of uniform films of vitreousselenium has required highly involved and critical processes,particularly processes involving the preparation of extremely clean anduniform substrates and vacuum evaporation techniques. This, togetherwith the high cost of selenium itself has led, by commercial necessity,to the use of selenium xerographic plates in repetitive processingcycles, that is, it required that the selenium plate be re-used manytimes in the xerographic process, so that the cost per copy of such aplate may be a reasonably small figure. Under conditions of optimum usea vitreous selenium plate can be used to prepare 100,000 or even morecopies before it deteriorates to the point of unsatisfactory imageformation. Under other conditions far fewer copies can be made.

The deterioration observed in selenium plates follows from themechanical abrasion attendant to the developing process and the cleaningstep wherein a rapidly rotating fur brush contacts the selenium surfaceto remove from the surface any developer particles adhering theretoafter the transfer step. In addition to mechanical abrae sion, the heatto which the plate is subjected both by virtue of the friction involvedin the various processing steps and, more important, by the propinquityof heat fusing devices generally located (by engineering necessity) inclose proximity to the xerographic drum in commercial machines.

In US. 2,663,636, Arthur Middleton disclosed various methods and meanswhereby any photoconductive insulating material in an insulating resinbinder can be formed into an operable xerographic plate. This discoverygreatly simplified the procedures for the preparation of xerographicplates and in particular eliminated the necessity for vacuum evaporationprocesses and for careful cleaning of the substrate.

Following this, Dr. Middleton, together with Mr. Reynolds found that axerographic plate can be prepared by intimately mixing together aphotoconductive material with a high resistance hinder, thephotoconductive material containing a metallic ion-containing inorganiccrystalline compound having electrons in the non-conductive energy levelactivatable by illumination to a different energy level whereby anelectric charge is free to migrate under an applied electric field.

This invention is disclosed in detail in U.S. Patent 3,- 121,006 whichwas copending with the parent of this application. An essential elementof this invention was the recognition that photoconductive materialscould be em ployed having substantially lower resistivities thanheretofore required. Thus, whereas prior to the work of Middleton andReynolds, it was understood that a xerographic coating must comprise aphotoconductive insulator meaning a material which itself could bothsupport an electrostatic charge in the dark and disperse the charge uponexposure to light, these workers recognized for the first time thatthese were necessary properties only of the composite layer of pigmentand binder, and that the individual pigment materials utilized in'suchstructures must be photoconductive as previously described, but maypossess a much wider range of resistivity than had been previouslyrecognized and still be operable in a xerographic binder plate. Inparticular, materials having too low a resistivity to support anelectrostatic charge in the dark when applied as a homogeneous layer toa conductive backing can be admixed with an electrically insulatingbinder and still function in the xerographic process.

The use of a binder did not extend the limits of the xerographic processto the point where all photoconductive, semiconductors and similarmaterials could be utilized in the preparation of xerographic binderplates, 1.e., it was still essential that the finely-dividedphotoconductive material incorporated in the binder have a resistivityin the range normally associated with insulators but no longer limitedto the extremes of resistivity as had previously been the case. Further,by the use of metallic ion-containing compounds, Middleton and Reynoldstaught that such materials by reason of their ability to vary bothoverall sensitivity and spectral response as by adding additional ionsto the lattice (i.e., doping) or by creating deficient states therein,so that the electronic structure is modified, permitted one to alter thephotoconductive properties of the compound. Thus, the flexibility ofpreparation discovered by Middleton, Middleton and Reynolds contributeda vastly Wider range of materials than heretofore and further taughtthat the preferred subclass of these materials could be modified so asto obtain virtually any desired spectral response and/or speed in theresulting xerographic plate.

These plates, i.e., the binder plates of Middleton and Reynolds, ingeneral, possessed physical properties inferior to vitreous seleniumplates, i.e., those pigments or photoconductive materials possessingoptimum photographic speed and spectral response were equally expensiveas vitreous selenium, while the resulting resin-pigment layers generallylacked the physical hardness for use under long processing cycles underthe conditions existing in commercial xerographic machines as abovedescribed.

For several years now there have been known or availableelectroluminescent cells, photocells, and other devices of similarstructure comprising a phosphor or photoconductive material embedded ina glass binder and having a conductive electrode on each surfacethereof, so that a current may be passed through the glass-phosphorcombination. Such devices are described for example in U.S. 2,930,999 toJ. G. Van Santen et al.; U.S. 2,993,001 to Schonebarger; U.S. 2,689,188to Hushley and U.S. 2,937,353 to Wasserman. These devices, requiring anohmic contact with conductive electrodes on each side of the layer, areantithetical to the properties of a Xerographic plate which requires ablocking contact on one surface and a free surface on the other for theformation and utilization of a stable electrostatic image and whichfurther requires resistivities many orders of magnitude higher than aretolerable in a photocell and the ability to accept a substantial chargeby corona depositon. Accordingly, such layers have not heretofore beensuitable for use in xerography.

I have now found that xerographic plates having a structure similar tothe binder plates of Middleton and Middleton & Reynolds, as discussedabove, may be prepared from the photoconductive materials known to thoseskilled in the art by mixing such finely divided photoconductiveinsulating materials with a glass enamel and fusing the enamel to aconductive backing to form a uniform layer of the photoconductiveparticles embedded in the glass binder.

The base or backing material used in preparing xerographic binder platesaccording to the invention provides physical support for aphotoconductive insulating layer and also acts as an electrical groundthereby permitting the photoconductive layer to receive an electrostaticcharge in the dark and permitting the charges to migrate when exposed tolight. It is evident that a wide variety of materials may be used, forexample, metal surfaces such as aluminum, brass, stainless steel,copper, nickel, zinc, etc.; conductively coated glass as tin or indiumoxidecoated glass, aluminum-coated glass, etc.; under certainconditions, as at higher temperatures, common plate glass has asufliciently low resistivity to act as a ground plane. In general, toact as a ground plane as described herein, a backing material may have asurprisingly high resistivity such as or 10 ohms-cm. The material must,however, be capable of withstanding the temperatures required for fusingthe glass enamel.

The photoconductive materials useful in the instant in.- vention are anyof those materials disclosed in the prior art as useful in xerographicbinder plates. A thorough discussion of these materials is given in S.N.668,165, now U.S. Patent No. 3,121,006, and is incorporated herein. Ingeneral, as there stated, a material is considered a photoconductorsuitable for use in a binder plate if it shows a resistivity in the darkabove about 10 ohms-cm. and a lower resistivity when exposed to light.Generally, photoconductive materials are described as beingcharacterized by having electrons in the non-conductive energy level(valence band) activatable by illumination to a different energy level(conduction band), whereby an electric charge is free to migrate underan applied electric field in the order of at least 10 volts per cm. Ingeneral, the composite resistivity of the photoconductive material inthe binder in the case of the instant invention should be at least about10 ohms-cm. in the absence of illumination. The materials which havebeen found useful in xerographic binder plates include withoutlimitation calcium-strontium sulfide, zinc sulfide, zinc oxide, zincselenide, cadmium sulfide, cadmium selenide, mercuric sulfide, antimonysulfide, arsenic sulfide, lead monoxide, gallium selenide, indiumsulfide, arsenic selenide, mercuric oxide, titanium dioxide, zinctitanate, zinc-magnesium oxide, zinc silicate, red lead, etc.

A particularly preferred class of photoconductive compounds are thosemetallic ion-containing inorganic compounds termed phosphors. By theterm phosphors as used herein is meant not only those metallicion-containing inorganic compounds which inherently or by virtue ofparticular methods of preparation, doping, etc., displayphotoluminescence when exposed to low energy photons, that is visiblelight or ultra-violet light, but any substance capable of luminescence.Luminescence is not, per se, desirable in this invention but isfrequently associated with photoconductivity. Particularly preferredmaterials are appropriately doped chalcogenides of zinc and cadmium and,even more particularly, the sulfides and selenides of these metalseither as mixed sulfides and selenides of zinc and/or cadmium, as amixed zinc and cadmium sulfide or selenide or as simple compounds.

Suitable photoconductive materials are available from various sources.They are often sold specifically as pigments, photoconductors, orphosphors. A suitable zinc oxide material, for example, is availablefrom the New Jersey Zinc Company as Florence Green Seal No. 8. Suitableactivated cadmium sulfide photoconductor materials are available fromRadio Corporation of America as F-2103 and F-2111. A suitableluminescent grade cadmium sulfide is also available from the GeneralElectric Company as type 118-8-2 and from Sylvania Electric Products.These materials are generally doped with an activator such as copper orsilver and aco-activator such as chlorine in order to achieve maximumphotosensitivity. If suitably doped materials are not available they maybe prepared by diffusing the activators into the basic materials underhigh temperature vacuum conditions or in a high temperature and pressurehydrothermal process. Further information on doping procedures may befound in U.S. Patent 2,876,202 and in the RCA Review for March 1959.

Cadmium sulfoselenides are useful materials in the present invention andmay be purchased or prepared. A bright orange red pigment F-14854 and amaroon pigment 1 -14857 from the Ferro Corporation are very suitable.They are known as cadmium oxide colors in the enamelling trade but areactually cadmium sulfoselenides. They also include substantial amountsof a glassy phase including A1 0 and SiO These additional constituentsappear to be beneficial rather than detrimental since they increase thecompability of the pigment with glassy binder materials. Cadmiumsulfoselenides may also be prepared by reacting elemental selenium withcadmium sulphide or by reacting sulphur and selenium with CdCO In apreferred method a mixture of about 4 parts cadmium sulfide to 1 partselenium is sealed in a glass container having a small vent opening andis heated to about 480 C. Another preferred method is to sinter amixture of finely divided CdS and CdSe in the presence of a smallpercentage of cadmium chloride as a flux.

A great variety of known glass-forming mixtures may be used as thebinder material. In general, there are three types of oxides used inmaking frits: acidic, basic and neutral or amphoteric. The acidicoxides, mainly SiO and P 0 are network forming and raise viscosity andmelting point when in excess. Less acidic or neutral oxides, such as B 0Sb O and As O do not raise viscosity and melting point; in fact, B 0actually lowers viscosity. The basic oxides such as Na O, CaO, K 0, MgO,BaO, PbO, ZnO and OdO are network stoppers and they lower viscosity andmelting point by making the glass network of oxygen bridges lessextensive. Fluoride is a unique acidic constituent with its viscositylowering property. Silica, the least soluble glass, also has the highestviscosity or softening point. As basic oxides are added to it, meltviscosity is lowered.

The main criteria of a desirable frit for embedding photoconductors tomake a xerographic plate are low fusing temperature needed to producefusing and inertness in forming poisoning by-products by reaction withthe photoc-onductor. The oxides contributing most to low fusingtemperature are B 0 and PbO, sodium oxide and potassium oxide. Fluoridesalso lower melting temperature but also cause silica and boric oxidevolatility when used in excess. Calcium oxide, and especially zinc oxideand cadmium oxide lower the softening point to a certain degree up to acertain extent. Antimony and arsenic oxides lower the melting pointalso. A typical frit may include from about 50 to 75 mol percent ofcombined B 0 and SiO and the rest basic oxides. Those metals which formblack sulfides tend to poison some photoconductors. In the case of leadoxide, however, and also iron, nickel and similar ions, this poisoningeffect may be overcome for a sulfide photoconductor by the addition ofcadmium oxide or other materials which prevent the gross formation oflead sulfide from the cadmium sulfide and lead oxide or borate byreversing the equilibrium.

Other metallic oxides conventionally used in glass formulations and inparticular those known as useful in matrix glass formulations such aslithium oxide, aluminum oxide, titanium dioxide, etc., may be present atleast in small amounts. While the glasses are generally considered interms of oxides, it is understood that low melting glassforming sulfidessuch as arsenic sulfide, antimony sulfide, etc., may also be used.

Typical compositions of frits useful as the binder in preparing theglass binder plates of the instant invention have the followingcomposition ranges (all figures are mol percentages) TABLE I 40-75combined SiO 0-50 CaO Zno was combined CdO PbO

N320 K 0 O-20 combined Li O NaF 0-10 A1 0 0-5 Sb O O- AS203 0-3 This isnot a hard and fast range of compositions, as is obvious to thoseskilled in the art. Thus Schonebarger, above, discloses a glass binderwhich is operable to make xerographic plates but is outside the rangesset forth above. However, the ranges set forth are an accurate guide informulating operable glass binders.

For example, a typical composition within this range,

useful in the instant invention, has the following composition:

An unusual property observed in the instant invention is that solderglasses, which consist almost entirely of lead borate, are useful informulating operable xerographic binder plates. Despite the knowndeleterious effect of lead when used without a stabilizing agent, suchsolder glasses may be used when they are heated sufiiciently to bond thephotoconductive pigment by fusion of the glass binder.

Commercially available frits useful in the preparation of xerographicbinder plates include Corning No. -2, a thermosetting solder glass;Corning 1970 and 1971 E. L., the latter two being electroluminescentphosphor embedding glasses, all available from the Corning Glass Co.;Dupont J-232 and N845, both of these materials being porcelain enamelfrits for use on aluminum; Harshaw fluxes AG 850, 862 and 881, all ofthese materials being enameling glasses for use on glass substrates andavailable from the Harshaw Chemical Company; and Al-8, a glass enamelintended for use on an aluminum base and available from the FerroCorporation. This material is furnished in a red pigmented form and isalso available in a clear unpigmented form. This is a proprietarymaterial, but it was analyzed spectroscopically and chemically, as shownin Table III below. The spectrographic analysis was done without benefitof standards so that much more confidence should be placed in thechemical analysis. Care should be taken not to overfire the J-232material, as it will become less resistive and unable to retain a coronacharge. If the glass will not retain a charge it is not operable to makea xerographic plate.

TABLE III Spectroscopic Analysis Chemical Analysis 1 Major.

Particularly where the photoconductive insulating layer is rather thickit is desirable that the heat expansion coefficients of the support andthe layer be roughly comparable. This condition is usually met by usingcommercial frits on the type of base material for which they areintended. It is understood that commercial frits for use with aluminumhave their coefficients of expansion most nearly matched to the 6000series of aluminum alloys, particularly 60614.

The relative proportions of binder and photoconductor are criticallyimportant in determining the operability of the structure in thexerographic process. In general, the photoconductor should comprise nomore than about 60%, by weight, of the total composition, and,preferably, from about 6 to about 20%. This is in marked contrast to theelectroluminescent cells and photo-cells of the prior art which arevirtually inoperative at these high binder to pigment ratios. It isbelieved that the higher binder-pigment ratio is the critical factor inestablishing a blocking contact between the photoconductive particlesand the electrically conductive backing plate which is believed to beessential for the operation of a xerographic plate of this type, while alow-binder photoconductor ratio apparently promotes a too close contactamong particles, and between particles and backing and establishes anohmic or injecting contact. Regardless of theory, it has been found thatthe above proportions must be adhered to in order to produce afunctioning xerographic plate capable of relatively retaining ordissipating a surface charge.

The thickness of the photoconductive insulating layers of the instantinvention is not critical and may vary from about 10 microns to about200 microns, It is preferred that the layers be from about 20 to about150 microns thick. These photoconductive insulating layers of theinvention are characterized by outstanding wear-resistance properties.

At the lower binder-pigment ratios found operable for xerographicplates, the surface of the plate may have a matte appearance.Accordingly, it may be desirable in achieving a glossy surface, toovercoat the surface of the plate either with a layer of clear glassbinder or with a layer of glass binder having a small amount ofphotoconductive particles therein, i.e., a substantially smaller amountof the photoconductor than in the main photoconductive insulating layer,to thus achieve a glossy surface. Alternatively, a glossy surface may beachieved by contacting the free surface of the photoconductive layerwith a very smooth surface while the photoconductive insulating layer isstill in a plastic or even molten condition. In this instance, it isimportant to select a smooth surface which is not adhesive to thephotoconductive insulating layer, so that there can be no adverseadhesion therebetween which would complicate separation of the surfacesafter the photoconductive insulating layer has been adequately smoothed.

The means of application of the photoconductorbinder combinations of theinvention are well known and are not critical in the instant invention.The glass binder may be used as received, or, if not sufiiciently finelyground, may be subjected to further processing as by ball-milling toproduce a smaller particle size of the glass particles. In general, theglass particles or frit should be no more than about 4 microns indiameter. However, the size of the particles may vary, depending on theviscosity of the resultant glass melt; the lower the viscosity, thelarger the particle size which may be tolerated while still achieving auniform layer. Similarly, the photoconductive insulators themselvesshould be in a suitably finely divided state. While photoconductiveparticle sizes as large as about 50 microns may be used, it is preferredthat particle size be as small as possible; in genenal, particle sizesof no more than [about 20 microns are used, and preferably thephotoconductive particles should have an average particle size of nomore than about 1 micron.

The finely divided photoconductive particles and glass binder particlesare desirably dispersed in a liquid, as

distilled water, or an organic liquid, as alcohol, ethyl acetate,ethylene glycol, etc. and a uniform dispersion obtained by mixing theliquid. In accordance with conventional enamelling practice thephotoconductive and glass particles may be ground together in waterusing small amounts of sodium silicate, sodium hydroxide and boric acidas dispersing agents. The resulting slurry may then be applied to thedesired surface by air spraying, dipping, painting, or other coatingoperation as is standard in the art. Care must be taken that air bubblesor other discontinuities are eliminated from the slurry prior tocoating. The coating is dried to remove most of the liquid. Then, beforecracking occurs, the plate is fired at the necessary temperature to fusethe glass binder and produce a uniform homogeneous layer ofphotoconductive pigment dispersed in a vitreous glass binder. Thesupport layer should be clean before the coating material is appliedthereto. Any conventional cleaning technique will sufiice. Aluminumlayers may simply be heated to the firing temperature and cooled before.the coating material is applied. However, the various known chemicaltreatments used in the enamelling art may also be used, if desired, asmay the controlled oxidizing treatment given conventional aluminum basedxerographic plates before selenium is applied thereto.

Having described the invention above, the following examples are givento more fully illustrate specific embodiments thereof. The examples aregiven for illustrativepurposes only and are not intended to be limitingon the scope of the invention. In the examples, all parts are byweigh-t, unless otherwise specified.

Example I Using distilled water, an aqueous slurry was prepared,containing 25% of a substantially non-luminescent zinc oxide pigment(obtained from New Jersey Zinc Company under the designation of FlorenceGreen Seal No. 8) and of a Du Pont mixed oxide frit designated J-232.The slurry was sprayed on an aluminum surface with an air brush, usingCO as the propellant. As soon as the water dried out, 'but beforecracking began, the plate was fired at about 1000" F. for about 5minutes. The plate was slowly cooled to room temperature and was thentested in the xerograp-lnc process. It was found that the plate acceptedan electrostatic charge in the dark and discharged the electrostaticcharge upon exposure to light, thus demonstrating that the plate wasoperable in xerography.

' Examples Il-III In these examples a RCA F-2103 cadmium sulphide wasmixed with Corning -2 solder glass and water to form a slurry and spreadwith a doctor blade on tin oxide coated soft glass to give a filmthickness of 5 mils. The films were 'then fired for about 2 minutes at450 C. Example II contained 20% CdS by weight and Example III contained40%. Example II was about twice as fast as a comparison vitreousselenium plate and Example III was about five times as fast. Xerographicprints were made from each of these examples.

' Examples IV-XIV In these examples the plates were prepared by firstpreparing a slurry of the designated enamel and photoconductive pigmentin distilled water and then spraying the mixture on the designatedbacking material with an air brush, using CO as the propellant. Afterspraying on the enamel and drying, but before cracking began,- plateswere slowly moved into the firing oven, and maintained at the correcttemperature, in this case 1000 F. except for the AG-88l binder, which isfired at 1200 F. The plates were held at this temperature for sixminutes and then slowly cooled to room temperature and tested as shownin Table IV.

TABLE IV Photocon- Photoeon- Thickness Sensitivity, Example No.Substrate ductor Type doctor, Binder (Microns) V (Volts) percent percentGlass-SnO F-14854 30 A G881 80 590 50 Aluminum-3003-S. F-l4854 5 Al-8110 +220 4 d F-14854 10 A1-8 70 +670 3 .4

670 8 F-14854 Al-8 100 +620 18 V 1, 020 I 21 F-14854 32 AH; 90 +490 12-490 42 F-14857 5 AH; 90 +550 3 570 0.6 F-14857 10 Al-8 100 +950 7 97013 F-14857 15 Al-8 90 +490 25 190 72 F14857 Al-B 90 +ig XIII F-2111 20Al-8 110 +338 1.3 3 1.3 XIV F-2111 30 Al-8 110 +170 3 +260 4 Forpurposes of comparison a selenium plate tested under similar conditionshas a sensitivity of between 20 IIIX. They were fired to 1200 F. Acommercial selenium plate was used as a control.

and 30%. Sensitivity is measured as the percentage of charge lost afterexposure for ,5 second to a specified quantity of illumination. V is theinitialpotential accepted from a corona charging device. The coronacharging current was somewhat lower for Examples VII- XII than for otherexamples. Pigment concentrations of over 15% gave rough surface, poorlysuited for xerography, with poor adhesion. The cadmium sulphide pigmentshowed poor adhesion with the A1-8 binder. Better results with thispigment were had with the AG-881 binder, as shown in a later example.

All of the particlesin these examples were quite uniform in appearance.The plates in Examples XIII-XV had a matte surface, making them lesssatisfactory for xerography while the plate of Example XVI had a smooth,glassy surface and perhaps for that reason a relatively high residual. Dis the percentage of charge lost after 30 seconds in darkness.

Examples X XII-X XIII In these examples Example XX was duplicated,excepting that in Example XXII one half of the plate surface TAB LE VPhoto- Photo- Thickness, Sensitivity. Ex. N0. conductor conductor,Binder Microns V0 (Volts) Percent Type Percent XV F-14854 15 Al'8 120+300 29 965 48 XVI F-14854 25 Al-8 110 +620 14 730. 68 XVII. F-l4854 15AG-88I 40 +830 6 1, 180 25 Examples X VIII-XX I These plates wereprepared as in Examples II-IX. In each case tin oxide coated glass wasused for the substrate and the Harshaw AG-881 for the glass binder. Thepigment in each of these plates was a commercial cadmium sulfidephotoconductor obtained from RCA and designated F-2l11. Thephotoconductive layers on these plates had a thickness f b t 4 fl (100 is) h, and the overcoated portions of the plates. The results Plates soprepared were-tested :as described in Examples are set forth in TableVII.

TABLE VII Example N0. Va (Volts) Dan, Percent Sensitivity, ResidualPercent (Volts) XXII (Non-overcoated) 180 10 0 XXII (Overcoated)-.- -85010 71' 470 XXIII (Non-overcoated) 155 94 o XXIII (Overcoated)... +680 1535 +225 As already noted the Ferro Corporation Al-8 aluminum enamel isalso available in a red form. X-ray diffraction analysis of thismaterial showed that it contains about 46% cadmium sulfoselenide.Accordingly, plates were made from this material in accordance with theplate making procedure already described, except that no additionalpigment was added to the red binder material. These plates proved to bephotoconductive and exhibited electrical properties comparable to thosein which a photoconductive pigment was separately added to aphotoconductive frit. These plates also made xerographic prints ofunusually high quality.

On testing a glass enamel binder plate in the xerographic process underrepetitive cycling conditions wherein the plate is sequentially chargedand then flooded with a 4 watt blue fluorescent lamp, no significantincrease of residual voltage or of dark decay rate was observed. Afterover 700 cycles the dark decay value increased only from about 15% toabout and after almost 1400 cycles there was no further increaseobserved.

Xerographic plates made according to the present invention often exhibitpoor electrical properties when newly made but light sensitivity anddark decay characteristics generally improve markedly during the first24 hours after manufacture. It has also been observed at times thatplates which exhibit good electrical properties nevertheless producexerograplu'c images of poor quality with low resolution or even no imageat all. The image-form ing qualities of these plates can often begreatly improved by polishing their surfaces, as by bufiing with rouge.A

simpler and generally more elfective treatment involve applying water tothesurface of the plate. This may be done by immersing the plate inordinary tap water for about 15 minutes and then thoroughly drying underan infrared lamp. This generally produces a marked improvement in theability of the plate to produce a xerographic print. Distilled water aswell as certain other materials classified as Lewis bases also have abeneficial effect but none appears to be more effective than ordinarytap water. Acids, on the other hand, are to be avoided because they notonly impair the ability of the plate to form a xerographic image but mayeven damage the plate to the extent that it will not accept anelectrical charge or will retain only a negligible charge.

A glass enamel plate was also compared to a commercial selenium plate inan abrasion tester. In this unit a vitreous enamel plate and a seleniumplate were placed inside a revolving drum loaded with silica sand. Aftermore than A of a million cycles in the cylinder the plates were removedand examined. About 7 microns of seleniurn had been abraded away fromthe selenium plate, while no detectable loss in'thickness could be seenin the enamel plate, the only obvious change being a slight reduction ingloss. Thus the vitreous enamel xerographic plates have an operatinglife between 25 to 250 times greater than the selenium plates. Further,on using a glass enamel binder plate at a temperature of 50 C. nodeleterious effect on the xerographic process is .observed.

When the surface of a glass enamel plate is abraded to the point whereit becomes dull, the xerographic properties of the plate are impaired.These properties can be restored to those of a new plate by re-firingthe plate under substantially the same conditions as used when the platewas first made. This entails bringing the plate up to about the glasssoftening point temperature. This largely restores the gloss to theplate and also largely'restores its xero graphic properties.

For example a xerographic plate was fabricated from '14 parts by weightof a cadmium sulfoselenide photoconductor in 100 parts by weight of aglass making frit composed of 65' parts by weight PbO, 18 parts byweight S10 8.1 parts by weight B 0 and 7.8 parts by weight CdO. Thismixture was coated on a conductive subparticles'are cadmiumsulfoselenide.

been produced, a gradual degradation in print quality.

became apparent and eventually reached a point where large sections ofthe print were either not reproduced or became illegible. At this pointthe plate Was removed from the copier and refired by again heating tothe glass softening point. After cooling and replacement in the copier,the plate produced more thantwice again as many prints as it hadoriginally before the onset of print degradation. The prints produced bythe refired plate were at least equal in quality to those produced bythe plate immediately after initial production. When print degradationdid finally set in once again, a second refiring was carried out underthe same conditions resulting in another doubling of the number of goodquality prints produced to about 27,000 copies prior to degradation.The. third refiring produced an additional 27,000 good quality copies.Both this plate and a large number of additional test plates respondedin about the same way showing great improvement and rejuvenation evenafter a fourth refiring.

Plates according to the present invention are very durable and haveother advantageous properties based on the superior physical propertiesof these plates. These may also have superior electrical properties. Inparticular, the techniques of the present invention enable economicalproduction of xerographic plates incorporating certain cadmiumchalcogenides. These photoconductors can be used in other structures andother photoconductors can be used in the present invention but thesephotoconductors appear to be unusually compatible with glass binderplate structures. When used in such structures they result in a platewith an unusual spectrosensitivity which extends into the red region ofthe spectrum whereas most practical xerographic plates whether of thevitreous or binder type have been sensitive primarily in ductive all theway out to approximately ten thousand angstroms.

What is claimed is:

1. The method of increasing the image reproducing properties of axerographic glass binder plate having impaired image reproducingproperties, which comprises heating said plate to a temperature of atleast the fusing point of the glass binder material in said binderplate, said plate initially comprising a photoconductive layer composedof finely divided inorganic photoconductive particles dispersed in ahighly insulating fused glass binder, said photoconductive particlesconstituting less than about 60 percent by weight of saidphotoconductive layer. T

2. The method of claim 1 wherein the photoconductive layer has athickness up to about 200 microns.

3. The method of claim 1 wherein the photoconductive particles areselected from the group consisting of at least one chalcogenide of atleast one material selected from 4. The method'of claim 1 wherein thephotoconductive 5. The method of claim 1 wherein the photoconductiveparticles are cadmium sulfide. I

6. A method of restoring the xerographic properties of a xerographicglass binder plate degraded through xerographic use which comprisesheating said plate to a temperature of at least the fusing point of theglass binder material in said binder plate, said plate initiallycomprising a photoconductive layer having finely divided inorganicphotoconductive particles dispersed in a highly insulating fused glassbinder, said photoconductive particles constituting less than about 60percent by weight ofs aid photoconductive layer.

7. The method of rejuvenating a xerographic glass binder plate having aphotoconductive layer made up of less than about 60 percent by Weight offinely divided photoconductive particles dispersed in a highlyinsulating fused glass binder after its image-forming capability hasbeen deteriorated by extended use in a xerographic copier, comprisingheating said plate to at least the softening point temperature of saidglass binder and then cooling said plate to room temperature at a ratesufficiently fast to prevent devitrification of the glass binder.

8. The method of extending the life of a xerographic glass binder platehavin a photoconductive layer made up of less than about 60 percent byweight of finely divided photoconductive particles dispersed in a highlyinsulating, fused, glass binder, comprising abrading the surface of saidxerographic plate, heating said abraded plate to at least the softeningpoint temperature of said glass binder and then cooling said plate toroom temperature at a rate sufficiently fast to prevent devitrificationof the glass binder.

9. The method of restoring and improving the imagereproducing capabilityof a xerographic binder plate having been previously degraded throughextended use which comprises heating said plate to a temperature of atleast the softening point of the glass binder material in said binderplate, said plate initially comprising a photoconductive layer havingfinely divided inorganic photoconductive particles dispersed in a highlyinsulating, fused, inorganic nonphotoconductive glass binder, saidphotoconductive particles constituting less than about 60% by weight ofsaid photoconductive layer and then cooling said plate to roomtemperature at a rate sufficiently fast to prevent devitrification ofthe glass binder.

10. The method of xerographic reproduction comprising cycling axerographic glass binder plate made up of a photoconductive layer on aconductive substrate, said photoconductive layer being composed offinely divided inorganic photoconductive particles dispersed in a highlyinsulating fused glass binder with said photoconductive particlesconstituting less than about by weight of said photoconductive layerthrough a xerographic reproduction cycle which includes charging saidplate, exposing said plate to an image to be reproduced to form a latentelectrostatic image, depositing fine electroscopic material on saidlatent image to form a developed image, and transferring said developedimage to a transfer layer until the image reproducing properties of saidplate are substantially impaired by said cycling and then heating saidxerographic glass binder plate above the softening point of the glass,cooling said Xerographic glass binder plate to room temperature at arate sufficiently fast to prevent the devitrification of the includedglass, and then again recycling said plate through said xerographicreproduction cycle.

References Cited by the Examiner UNITED STATES PATENTS 1,667,146 4/1928Drake 61 2,371,486 3/1945 Walker 65-61 2,573,200 10/1951 Hushley.

2,930,999 3/ 1960 Van Santen et a1. 252501 X 2,937,353 5/1960 Wasserrnan252501 X 3,077,398 2/1963 Jones 961 3,151,982 10/1964 Corrsin 96-1NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner.

1. THE METHOD OF INCREASING THE IMAGE REPRODUCING PROPERTIES OF AXEROGRAPHIC GLASS BINDER PLATE HAVING IMPAIRED IMAGE REPRODUCINGPROPERTIES, WHICH COMPRISES HEATING SAID PLATE TO A TEMPERATURE OF ATLEAST THE FUSING POINT OF THE GLASS BINDER MATERIAL IN SAID BINDERPLATE, SAID PLATE INITIALLY COMPRISING A PHOTOCONDUCTIVE LAYER COMPOSEDOF FINELY DIVIDED INORGANIC PHOTOCONDUCTIVE PARTICLES DISPERSED IN AHIGHLY INSULATING FUSED GLASS BINDER, SAID PHOTOCONDUCTIVE PARTICLESCONSTITUTING LESS THAN ABOUT 60 PERCENT BY WEIGHT OF SAIDPHOTOCONDUCTIVE LAYER.